[cite book |author=Pawlina, Wojciech; Ross, Michael W.; Kaye, Gordon I. |title=Histology: a text and atlas: with cell and molecular biology |publisher=Lippincott Williams & Wilkins |location=Hagerstown, MD |year=2003 |pages= |isbn=0-683-30242-6 |oclc= |doi=] The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent.] Matrix
Around the papilla is the hair matrix, a collection of epithelial cells often interspersed with the pigment producing cells, melanocytes. Cell division in the hair matrix is responsible for the cells that will form the major structures of the hair fiber and the inner root sheath. The hair matrix epithelium is one of the fastest growing cell populations in the human body, which is why some forms of chemotherapy that kill dividing cells or radiotherapy may lead to temporary hair loss, by their action on this rapidly dividing cell population. The papilla is usually ovoid or pear shaped with the matrix wrapped completely around it except for a short stalk-like connection to the surrounding connective tissue that provides access for the capillary.
Root Sheath
The root sheath is composed of an external root sheath (Henle's layer), a middle layer (Huxley's layer), and an internal cuticle that is continuous with the outermost layer of the hair fiber.
Hair Fiber
The hair fiber is composed of a cuticle that is continuous with the root sheath, an intermediate cortex, and an inner medulla.
Other Structures
Other structures associated with the hair follicle include arrector pili muscles, sebaceous glands and apocrine sweat glands. Hair follicle receptors sense the position of the hairs.
Hair growth phases
Hair grows in cycles of various phases: [cite journal
author=K. S. Stenn and R. Paus
title=Controls of Hair Follicle Cycling
journal=Physiological Reviews
year=2001
volume=81
issue=1
pages=449–494
url=http://physrev.physiology.org/cgi/content/full/81/1/449
pmid=11152763 ("comprehensive topic review, successor to landmark review of 1954 by HB Chase")] anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase. Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one of several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase while, 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hairs have a fixed length, while hairs on the head seem to have no length limit. Growth cycles are controlled by a chemical signal like epidermal growth factor.
Anagen Phase
Anagen is the active growth phase of hair follicles. [cite web
url=http://dermatology.about.com/od/hairanatomy/l/bldefanagen.htm
title=Anagen Phase
accessdate=2007-11-14
last=Brannon, MD
first=Heather
publisher=The New York Times] The cells in the root of the hair are dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2-7 years. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
Catagen Phase
The catagen phase is a short transition stage that occurs at the end of the anagen phase. [cite web
url=http://dermatology.about.com/library/bldefcatagen.htm
title=Categen Phase
accessdate=2007-11-14
last=Brannon, MD
first=Heather
publisher=The New York Times] It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while a club hair is formed.
Telogen Phase
The telogen phase is the resting phase of the hair follicle. [cite web
url=http://dermatology.about.com/od/glossaryt/g/telogen1.htm
title=Telogen Phase
accessdate=2007-11-14
last=Brannon, MD
first=Heather
publisher=The New York Times] During this phase the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. At any given time, 10–15% of all hairs are in the telogen phase.fact|date=August 2008 This phase lasts for about 100 days for hairs on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg.fact|date=August 2008 About 25–100 telogen hairs are shed normally each day.fact|date=August 2008
Hair growth cycle times
*Scalp: The time these phases last varies from person to person. Different hair colour and follicle shape affects the timings of these phases.
**anagen phase, 2–3 years (occasionally much longer)
**catagen phase, 2–3 weeks
**telogen phase, around 3 months
*Eyebrows etc:
**anagen phase, 4–7 months
**catagen phase, 3–4 weeks
**telogen phase, about 9 months
=Additional
References
